If this is what a World Cup hangover looks like, the rest of the tournament should be quite something. Northampton and Munster have a history of titanic struggles and this was another in the series: tense, unforgiving and unpredictable. For the umpteenth time, though, this was Ronan O'Gara's night, a 40-metre drop goal with the game's final kick after an incredible 41 phases, sneaking a victory from under Saints' disbelieving noses.

It was a cruel outcome for Ashton, Courtney Lawes and Ryan Lamb, all of whom gave influential performances, but Munster are never more dangerous than when they appear beaten. With John Hayes also becoming the first player in the history of the tournament to reach a century of appearances when he came on as a late replacement, it was another memorable evening for the home side.

It was also another controversial one from a refereeing perspective. The Northampton coach Jim Mallinder could not hide his frustration, driven to distraction by some of Nigel Owens' calls. "The game did hinge on a couple of crucial refereeing decisions," Mallinder said bluntly. "He just got quite a lot wrong." Top of the list was the crucial late scrum awarded to Munster after Northampton had driven a lineout at least 10 metres upfield. "You don't concede a scrum when you're going forward," said Mallinder, before departing to study the agonising video.

He had a point, but the contest fluctuated wildly from start to finish, with both sides enjoying their moments of fortune. Maybe Saints, still smarting from their second-half collapse to Leinster in the final in May, will find their luck changing at some stage, but this outcome will be mighty hard to take. It will be no consolation that two of the game's four tries, scored by the flying Chris Ashton and Doug Howlett, would have graced any game.

For the first time in ages there were still tickets available for a big Munster game on matchday itself, but some things never change. Despite a slightly unfamiliar looking Munster teamsheet, they started strongly and were gifted a softish try inside four minutes. What should have been a routine defensive lineout was too easily outwitted and Damien Varley thundered over beneath a heap of team-mates.

It was a reminder, as if it were needed, that Northampton would have to grasp the nettle rather than await donations. Soon enough they hit the accelerator, Vasily Artemyev releasing the enterprising Ben Foden in space on the left. The full-back has been as eager as anyone to make amends for his World Cup disappointment and expertly drew the cover to send Ashton hurtling beyond James Coughlan. This has not been a good week for Ashton, with a suspended £5,000 fine now hanging over him following the Rugby Football Union's dim view of off-field events in New Zealand, but at least his form is slowly returning.

Two finely-struck penalties from Lamb further extended Saints' lead to 13-7 early in the second quarter, although Munster would have had a second try had Niall Ronan's attempted scoring pass to Conor Murray not been rightly called forward. Even so, Northampton would have gone in ahead at the interval had a 65-metre raking punt from Lamb not bounced unkindly dead.

Munster seized greedily on the opportunity, O'Gara slotting a scrum penalty before Howlett showed why he was such a distinguished All Black winger. When he burst through with only Foden to beat, the full-back would not have expected to barely get a hand on his opponent. Two seconds later, the Kiwi was under the sticks and, unexpectedly, Munster went in 17-13 ahead.

A penalty apiece for O'Gara and Lamb maintained the tit-for-tat theme, but the Northampton coaches clearly feared the worst when Lamb missed a vital touch from a penalty. The expression on Dorian West's face was that of a man who wanted to haul the fly-half off the field there and then but, fortunately for Saints, Lamb was reprieved. His next act was to throw a suspiciously forward-looking pass that bounced off O'Gara's arm and back into Saints' arms, launching an attack that ended in a try for Downey. It summed up a game of breathless intensity and rollercoaster intrigue.

For Northampton, though, it was ultimately a huge missed opportunity. Had Munster lost, a second defeat to Castres next weekend would have effectively counted them out of the tournament. Instead, to Mallinder's intense annoyance, they were denied by the most reliable cat burglar in the European business. O'Gara, unlike England's fly-halves in New Zealand, was content to wait his moment, trusting his forwards to steer him into the right position. It took a while but, when the moment came, O'Gara was triumphantly equal to the challenge.